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https://issues.apache.org/jira/browse/HDFS-4403?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13554527#comment-13554527
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Todd Lipcon commented on HDFS-4403:
-----------------------------------
bq. new client talks to old server: new clients infers the checksum type by
reading first byte
If you mark it 'required', then the new client will fail to deserialize a
response from an old server, due to the missing field. To verify this, I
changed my test protobuf above to make the field required, and tried to
deserialize a byte array which didn't include the field using
MyTestProto.parseFrom. It threw "InvalidProtocolBufferException: Message
missing required fields: testField"
BTW, the protobuf guide also talks about this specific case of changing the
default:
{quote}
Changing a default value is generally OK, as long as you remember that default
values are never sent over the wire. Thus, if a program receives a message in
which a particular field isn't set, the program will see the default value as
it was defined in that program's version of the protocol. It will NOT see the
default value that was defined in the sender's code.
{quote}
> DFSClient can infer checksum type when not provided by reading first byte
> -------------------------------------------------------------------------
>
> Key: HDFS-4403
> URL: https://issues.apache.org/jira/browse/HDFS-4403
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: hdfs-client
> Affects Versions: 2.0.2-alpha
> Reporter: Todd Lipcon
> Assignee: Todd Lipcon
> Priority: Minor
> Attachments: hdfs-4403.txt
>
>
> HDFS-3177 added the checksum type to OpBlockChecksumResponseProto, but the
> new protobuf field is optional, with a default of CRC32. This means that this
> API, when used against an older cluster (like earlier 0.23 releases) will
> falsely return CRC32 even if that cluster has written files with CRC32C. This
> can cause issues for distcp, for example.
> Instead of defaulting the protobuf field to CRC32, we can leave it with no
> default, and if the OpBlockChecksumResponseProto has no checksum type set,
> the client can send OP_READ_BLOCK to read the first byte of the block, then
> grab the checksum type out of that response (which has always been present)
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